Complete a series of questionnaires (study start, 12 months). These will include detailed questions about the foods you eat, your exercise and health habits, medical history and health status, reproductive and menstrual history, and quality of life (social support, thoughts and feelings). You may choose not to answer any question on any questionnaire.

Come for clinic visits (study start, 6 months, 12 months) at the Fred Hutch (FHCRC) Cancer Prevention Studies Clinic located in the Public Health Sciences Building on the FHCRC campus at Southeast Lake Union. These clinic visits will include: a brief physical exam and medical history, a fasting blood sample, a urine sample, body fat measurement, a muscular strength test, a DEXA scan and an optional biopsy of breast and abdominal fat tissue.

Take a daily study pill. A computerized program will assign you by chance (like the toss of a coin) to take either: i) a study pill that contains Vitamin D (2000 IU) or ii) a placebo (no active ingredient). For the duration of the trial, neither you nor the study staff will know which type of pill you are taking.

Participate in a Weight Loss Program that involves both a Nutrition and Exercise component:

Nutrition Component: will include extensive teaching about how to achieve healthy weight loss through calorie reduction. Teaching will take place during group sessions (30-45 min in length) that cover topics such as calorie counting, portion control, eating at restaurants, and overcoming common barriers to weight loss. Sessions will be held weekly for 6 months, then monthly for the second 6 months. You will also have one individual meeting with the study nutritionist to develop a personal weight loss goal. As part of the nutrition program, you will be asked to:

attend scheduled sessions (morning and evening sessions available)

set a calorie goal and overall weight loss goal of 1 to 2 pounds per week

have regular private weigh-ins

keep a daily food journal

Aerobic Exercise Component: will include exercising 5 days per week for 45 minutes per day, throughout the12 month program. As part of the exercise program, you will be asked to:

attend 2 exercise sessions per week at the FHCRC exercise facility where you will use aerobic machines (e.g., stationary bicycle, treadmill, elliptical trainer, or rowing machine) with supervision by an exercise specialist. Morning and evening sessions will be available.

exercise on your own at home (or at a gym/exercise facility) on 3 additional days per week.

wear a pedometer (provided) and record your daily steps in an exercise journal.